1. Field of the Invention
The present invention is directed to a system that simulates radar targets, and more particularly, to a system in which pulse repetition frequency (PRF) targets in excess of 500KHz can be simulated.
2. Description of the Related Art
Radar systems must be tested before they are installed in the field. To perform this testing, simulators are used to simulate dynamic target returns that are consistent with real world expectations. Each simulated target return presented to the radar must have the following timing characteristics: 1. A return pulse should be generated for each transmit pulse regardless of the rate or pulse repetition frequency at which the transmit pulses occur. 2. Each return pulse should be delayed from its associated transmit pulse by a
time T=2R/C where R is the target range and C is the speed of light. The longest delay, T.sub.max, must be greater than or equal to the delay for real world targets located at the maximum range at which the targets are still detectable by the radar. 3. The target must be able to move smoothly with respect to the radar. To move the target digitally, the delay T must be adjusted by an amount plus or minus .DELTA.T periodically where the size of the incremental delay .DELTA.T is determined by the radar range resolution. If, for example, the radar can resolve target positions to within 10 feet, then the delay change can be no larger than 20 nanoseconds where the speed of light is assumed to be one foot per nanosecond. Early target simulators used relay selectable delay lines to simulate the delay between the transmit pulse and the return pulse. Depending on relay selection, when n delay lines are provided, the operator could select up to 2.sup.n different delays for each return pulse If the delay lines used are coaxial cable, a very significant amount of coaxial cable is necessary because each nanosecond of delay requires approximately one foot of coaxial cable. One shot multivibrators, discreet delay devices and coaxial cable lengths provide delays typically having a tolerance of three percent. With a delay network providing an approximately two millisecond delay sufficient to simulate returns for a target at 10.sup.6 feet from the radar, the variance in the delay can be as high as plus or minus 60 microseconds or plus or minus 30,000 feet. As a result, delay line simulators are inadequate to simulate targets for current radar systems.
Current simulators use a range return counter approach as exemplified by U.S. Pat. No. 4,168,502. Such current simulators are generally limited to 8 KHz pulse repetition frequency (PRF) In the range return counter approach, a series of counters act as digital delay elements in the production of target returns as exemplified by the delay circuit 37 in FIG. 3 of the above-identified patent. A more detailed reproduction of such a return counter delay circuit 8 is illustrated in FIG. 1 herein The inputs of these counters 10-19 are connected together and receive the present range from the target to the radar. As each transmit pulse occurs one of the range return counters is preloaded with the range and counted down For example, for the first transmit pulse, channel one might be enabled, for the secord pulse, channel two might be enabled and etc. In this manner up to N returns may be in some stage of processing at the same time The total number of counters necessary for this approach is a function of the highest radar PRF and the maximum radar range where N =CELL[2PRF.sub.m (R/C)] with R being the maximum range, C the speed of light, PRF.sub.m the maximum PRF and N the number of return counters in operation at the same time The range return counter approach has the following drawbacks. In this approach, returns are generated only for those transmit pulses having PRFs equal to or below PRF.sub.m, thus if the maximum PRF is doubled in a future radar system, twice the current number of return range counters will be required Doubling the maximum range of the radar also doubles the number of counters A radar simulator with 16 range return counters can operate at PRFs up to 16KHz for targets up to 80 nautical miles away. A typical range return counter requires 8 integrated circuits. In a radar with a PRF over 100 Khz with targets ranging up to 160 nautical miles at least 200 return range counters are required in the simulator requiring approximately 1,600 integrated circuits The range counter approach thus has severe practical limitations.